Coating compositions for precoated steel sheets are required to have various properties, such as high hardness, good workability, stain resistance, chemical resistance, water-proofness and distinction of image. Of these, the hardness and the workability are important characteristics, which, however, are contradictory to each other but the levels of which are required to be high. The workability as referred to herein indicates that, when flat, precoated steel sheets are worked into various shaped structures through bending, cutting or drawing, the paint films are damaged slightly. Therefore, it shall be said that the larger the degrees of elongation and flexibility of paint films themselves, the better the workability of the paint films. In particular, the bending workability (flexibility) of paint films varies, depending on the thickness of the steel sheets coated with the films.
In the T-shape bending test (according to 12.2.2 in JIS G3312) which is generally used in evaluating the flexibility of precoated steel sheets, a precoated steel sheet to be tested is bent, with putting a plural number, n, of specimen plates all having the same thickness inside the sheet being bent, and the bent part of the sheet is observed under a magnifier as to whether or not the paint film on that part is cracked. In the test, the smallest number of the specimen plates as put inside the precoated steel sheet being tested with no paint film cracking is determined as 1T, 2T, and this indicates the tested steel sheet flexibility. In this T-shape bending work, the degree of elongation of the outermost periphery of the sheet being bent is represented by the following equation: EQU .epsilon.=(t/2)/(r+t/2)
wherein;
.epsilon.=elongation strain of the outermost periphery (%), PA1 t=thickness (mm), PA1 r=R(radius of curvature) at the inside of the bent part (mm). PA1 wherein n represents an integer of from 2 to 10; and R represents ##STR4## PA1 (a) 4,4'-diphenylmethane diisocyanate, isophorone diisocyanate, tolylene diisocyanate, xylylene diisocyanate, naphthalene-1,5-diisocyanate; PA1 (b) hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, hydrogenated 4,4'-diphenylmethane diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylylene diisocyanate. PA1 (a) 4,4'-diphenylmethane diisocyanate, isophorone diisocyanate, tolylene diisocyanate, xylylene diisocyanate, naphthalene-1,5-diisocyanate; PA1 (b) hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, hydrogenated 4,4'-diphenylmethane diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylylene diisocyanate. PA1 wherein n represents an integer of from 2 to 10; and R represents ##STR7## ii) a polyol except i); and iii) a curing agent of a mixture composed of a blocked polyisocyanate compound as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanates of the following group (a) and their derivatives, and a blocked polyisocyanate compound as obtained by blocking one or more polyisocyanate compounds selected from polyisocyanates of the following group (b) and their derivatives: PA1 (a) 4,4.sup.1 -diphenylmethane diisocyanate, isophorone diisocyanate, tolylene diisocyanate, xylylene diisocyanate, naphthalene-1,5-diisocyanate; PA1 (b) hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, hydrogenated 4,4'-diphenylmethane diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylylene diisocyanate.
Accordingly, where precoated steel sheets as tested in the T-shape bending test give the same value, those having a thinner thickness shall have a larger elongation strain (%) at their outermost periphery and have a smaller curvature at their bent part, indicating that they have good workability.
To evaluate the workability of precoated steel sheets, the T-shape bending test noted above is generally employed. However, stamping and roll forming which are generally effected in industrial production of household electric appliances by manufacturers differ from the working in the T-shape bending test in the working history and the working speed, and the former require severer working conditions than the latter. The working history includes the working steps up to the final working level. For precoated steel sheets worked up to the same final working level, those as worked to have a large value R in the initial working stage shall receive a small degree of stress at their paint films as the strain resulting from the initial working may disperse throughout the part as bent to have the value R, while those as worked to have a small value R in the initial working stage shall receive a large degree of stress at their paint films as the strain resulting from the initial working may concentrate in the limited area of the part as bent to have the value R. Therefore, as compared with precoated steel sheets tested in the T-shape being test comprising U-shape bending followed by contact bending, those as tested by V-shape bending followed by contact bending shall receive a larger degree of stress at their paint films even though they are all worked up to the same final working level. Accordingly, the latter testing method is useful for simple evaluation of severe working.
On the other hand, it is desirable that the hardness of paint films is higher in order to make them have higher durability, while paint films having higher hardness tend to have lower workability.
In general, precoated steel sheets are required to have good corrosion resistance and good scratch resistance. The corrosion resistance indicates that, while the shaped articles of precoated steel sheets are used in the environment, the paint films do not peel off and the articles are rusted little or are not rusted at all. The scratch resistance indicates that, while precoated steel sheets are worked, they are hardly scratched due to friction with the parts of the working machine used to have shaping faults, and while the shaped articles of precoated steel sheets are conveyed, they are hardly scratched to have handling faults.
Regarding coating compositions for precoated steel sheets, polyester resins and epoxy resins have heretofore been used essentially in prime coating compositions for outside and in coating compositions for back sides, while polyester, acrylic or polyvinyl coating compositions in topcoating compositions for outside. Polyester resin coating compositions comprising a combination of polyester resins (main ingredients) and melamine compounds (curing agents) are the most typical as topcoating compositions for outside, in which the polyester resins give the workability and the melamine compounds give the hardness. However, their properties are not always satisfactory when they are used in precoated steel sheets which are required to have the highest level properties with severity, especially in those for household electric appliances, and therefore, the improvement in the properties of such topcoating compositions for outside is required.
For these requirements, various proposals such as those mentioned below have heretofore been made. For example, in Japanese Patent Application Laid-Open (JP-A) No. 6-234752, the film formation is tried by reacting a polyester resin with a melamine resin (curing agent) as modified to have orientability. They say that the coating film has high flexibility and high hardness.
In JP-A No. 7-233349, proposal is a coating composition for precoated steel sheets, which comprises a polyol as the essential ingredient and an isocyanate compound and/or an amino resin as the curing agent, to which is added a polyester compound as obtained by reacting essentially naphthalene-2,6-dicarboxylic acid and/or its lower alkyl ester with an alcohol component. The coating composition is intended to give a coating film having good workability and high hardness.
In JP-A No. 7-316497, proposal is a coating composition for precoated steel sheets, which comprises a polyol as the essential ingredient and an isocyanate compound and/or an amino resin as the curing agent, to which is added a polyester compound as obtained by reacting essentially 1,4-cyclohexane-dicarboxylic acid and/or its lower alkyl ester with an alcohol component. The coating composition is intended to give a coating film having good workability and high hardness.
In JP-A No. 7-316498, proposal is a coating composition for precoated steel sheets, which comprises a polyol as the essential ingredient and an isocyanate compound and/or an amino resin as the curing agent, to which is added a compound having a specific structure of the following chemical formula: EQU HO--(CH.sub.2).sub.n --O--R --O--(CH.sub.2).sub.n --OH
wherein n is an integer of from 2 to 10, and R represents ##STR1## The coating composition is intended to give a coating film having good workability and high hardness.
Of those prior art techniques, one disclosed in JP-A No. 6-234752 is problematic in that, at present, it is difficult to produce practicable modified melamine resins on an industrial scale as the process for producing the resins is long.
The others disclosed in JP-A Nos. 7-233349, 7-316497 and 7-316498 are also problematic in that they are ineffective in improving the properties, including the stain resistance, of the coating films formed. Therefore, even though both the hardness and the workability of the coating films are good, the stain resistance thereof is poor; or if the hardness and the stain resistance of the coating films formed are good, the workability thereof is poor. Anyhow, according to the techniques disclosed, it is impossible to obtain coating films that satisfy all the requirements, hardness, workability and stain resistance.
To improve the scratch resistance of precoated steel sheets while they are worked, wax (lubricant) may be added to the coating films. Wax for this use includes, for example, natural wax such as carnauba wax and palm oil, and synthetic wax.
However, the conventional coating films containing such a lubricant are problematic in that (1) wax or the like lubricant existing on the surface of the films is peeled off, when contacted with other substances, thereby soiling the surface of the objects contacted with the films, (2) it coagulates, when contacted with water, thereby fogging the surface of the films, and (3) it forms cissing faults and eye holes on the surface of the films.
Given the situation, accordingly, one object of the present invention is to solve various problems in the prior art such as those mentioned hereinabove, and to provide a coating composition for precoated steel sheets which can form a coating film having both good workability and high hardness and additionally good stain resistance even on relatively thin steel sheets.
Another object of the invention is to provide a precoated steel sheet, of which the coating film has high hardness and good stain resistance and additionally such good workability that the coating film is hardly cracked even under severe working conditions.
Still another object of the invention is to provide a precoated steel sheet, of which the coating film has not only high hardness, good workability and good stain resistance, but also good corrosion resistance.
Still another object of the invention is to provide a precoated steel sheet, of which the coating film has not only high hardness, good workability and good stain resistance but also good scratch resistance, and is not fogged on its surface by coagulation of wax.
Still another object of the invention is to provide an inexpensive and efficient method for producing those precoated steel sheets noted above.